1. Field of the Invention
The present invention relates to a laminated coat-type magnetic recording medium, particularly relates to the improvement of its characteristics of electromagnetic transduction and its durability in running in a high density mass storage recording area.
2. Description of the Related Art
For a magnetic recording medium, a so-called coat-type magnetic recording medium the magnetic layer of which is formed by coating a nonmagnetic base material with magnetic coating which is prepared by dispersing ferromagnetic powder, binder and a variety of additives together with an organic solvent and drying it is known and for the above ferromagnetic powder, a metallic particulate is used for densifying recording.
A coat-type magnetic recording medium using such a metallic particulate is used as a recording medium for a computer such as a high density floppy disk and a data cartridge for backup in addition to audio or video magnetic tape, forms a mainstream of a magnetic recording medium at present and the characteristics have been also remarkably enhanced.
To densify recording on a coat-type magnetic recording medium, it is important to smooth the surface of a medium as much as possible so as to minimize vain space and to reduce the loss of output due to demagnetization in addition to using a metallic particulate as ferromagnetic powder.
For a method of achieving these purposes, (1) increasing the coercive force and saturation magnetization of ferromagnetic powder, (2) unifying the distribution of the coercive force of ferromagnetic powder, (3) providing vertical anisotropy and (4) thinning a magnetic layer can be given.
Methods (1) and (2) of these are ones of increasing output directly. For such improvement of coercive force and saturation magnetization, the composition of an element constituting ferromagnetic powder is examined, and a metallic particulate the coercive force of which exceeds 160 kA/m and further, a metallic particulate the saturation magnetization of which exceeds 140 Am.sup.2 /kg are developed. The distribution of coercive force is related to the particle size distribution of ferromagnetic powder and is also remarkably improved by unifying the size of a particle.
The method of providing vertical anisotropy (3) is one for densifying vertical magnetic recording. In the case of a coat-type magnetic recording medium, vertical anisotropy is provided mainly by controlling the magnetic orientation of ferromagnetic powder. For example, if an acicular particle is used, vertical orientation processing or oblique orientation processing is tried for a coating film. However, such orientation processing is not made practicable yet because of a problem such as the difficulty of controlling orientation and the turbulence of the surface of a coating film by orientation.
Next, for a method of thinning a magnetic layer (4), it is conceivable that it is very effective to reduce loss by self-demagnetization.
If a magnetic layer is simply thinned until it is 1 .mu.m or less thick for example, the shape of the surface of the nonmagnetic base material readily emerges on the surface of the magnetic layer and the surface of the magnetic layer cannot be smoothed. Therefore, if a magnetic layer is thinned, a nonmagnetic coating layer is often inserted between a nonmagnetic base material and a magnetic layer. The thickness between the surface of the nonmagnetic base material and the surface of the magnetic layer is increased by inserting the nonmagnetic layer as described above and the shape of the surface of the nonmagnetic base material seldom emerges on the surface of the magnetic layer. Therefore, a thin magnetic layer with the smooth surface is formed.
For such a laminated coat-type magnetic recording medium, a variety of improvements are proposed and for example, a method of forming the coating of the lower nonmagnetic layer so that it is 0.5 to 3.5 .mu.m thick which is disclosed in Japanese published unexamined patent application No. Sho63-187418, a method of adding a proper quantity of carbon black in the lower nonmagnetic layer which is disclosed in Japanese published unexamined patent application No. H4-238111, a method of coating the surface of the nonmagnetic oxide of the lower nonmagnetic layer with an inorganic matter which is disclosed in Japanese published unexamined patent application No. H5-182177, a method of using two types or more of nonmagnetic powders which are different in size of a particle for the lower nonmagnetic layer which is disclosed in Japanese published unexamined patent application No. H5-274651, a method of limiting the standard deviation of the thickness of the upper magnetic layer in the specific range which is disclosed in Japanese published unexamined patent application No. H5-298653 and a method of constituting the upper magnetic layer by two or more magnetic layers which is disclosed in Japanese published unexamined patent applications No. H6-162485 and No. H6-162489 are reported.
A method of forming the lower nonmagnetic layer and the upper magnetic layer is also examined and a simultaneous laminated coating system (a wet-on-wet coating system) in which nonmagnetic coating and magnetic coating are simultaneously applied on a nonmagnetic base material using a die head provided with two slits through which nonmagnetic coating and magnetic coating are respectively pushed out is proposed. According to this simultaneous laminated coating system, a coating film of uniform thickness with few defects or unevenness can be formed. Therefore, a medium with little noise which is excellent in an electromagnetic transduction characteristic can be obtained. The adhesion between formed upper and lower layers is high and excellent durability can be obtained.
For this simultaneous laminated coating system, it is important to adjust the characteristics of upper and lower coating. From such a viewpoint, a method of using a solvent poorer than binder as a solvent used for preparing coating for upper and lower layers which is disclosed in Japanese published unexamined patent application No. Sho63-31028, a method of equalizing parameters of the solubility of coating for upper and lower layers which is disclosed in Japanese published unexamined patent application No. H3-119518, a method of equalizing Reynolds numbers of coating for upper and lower layers which is disclosed in Japanese published unexamined H4-271016, a method of providing equal or substantially equal degree of thixotropy to coating for upper and lower layers which is disclosed in Japanese published unexamined patent application No. H4-325917, a method of fitting the flow curve of coating to a specific expression which is disclosed in Japanese published unexamined patent application No. H5-128496, a method of specifying a flow index when coating is spread which is disclosed in Japanese published unexamined patent application No. H5-208165, a method of specifying the creep deformation quantity of coating which is disclosed in Japanese published unexamined patent application No. H6-195690 and a method of fixing the ratio of the maximal and minimum values of the loss elasticity of coating which is disclosed in Japanese published unexamined patent application H5-266463 are proposed.
As the surface of a laminated coat-type magnetic recording medium is formed so that it is very smooth, the area which is in contact with a variety of sliding members when the recording medium is run on a recording reproducer is large and the coefficient of friction with these sliding members is large. Therefore, it is difficult to obtain high durability in running. In addition, recently magnetic tap has a tendency to be thinned to extend the length of a tape which can be housed in a cassette and increase recording capacity per cassette. Therefore, it is more difficult to obtain high durability.
Therefore, a method of adjusting the quantity of lubricant used for an upper layer which is disclosed in Japanese published unexamined patent applications No. H1-224919 and No. H5-183178 and a method of using fluoric lubricant as lubricant which is disclosed in Japanese published unexamined patent applications No. H2-192018 and No. H5-298679 are also proposed.
As described above, for a laminated coat-type magnetic recording medium, a variety of improvements are proposed, however, particularly information related to the electromagnetic transduction characteristic in a high density recording area and durability in running in case such a recording medium is thinned is insufficient and examination related to them is further requested.